1. Field of the Invention
This invention relates to an expansion-controlled light alloy piston.
2. Discussion of Prior Art
Light alloy pistons for a conversion of thermal energy to mechanical energy have become firmly established in modern internal combustion engines. Such pistons must meet numerous requirements under all operating conditions. It is always desired that the light alloy pistons move exactly along a straight line although the coefficient of expansion of the piston material differs greatly from that of cylinders consisting of grey cast iron.
A concept underlying all piston designs of that kind is to provide expansion-inhibiting steel inserts, which prevent an undesired thermally induced increase in diameter of the piston and ensure that the piston clearances in the pressure and backpressure directions match the diameter of the cylinder under all operating conditions, as far as possible. That action is described as expansion control.
Under thermal load and gas pressures applied, the cylinder of an internal combustion engine deforms in radial and axial directions and hardly retains its cylindrical shape. The piston follows these deformations by necessity and must adapt itself thereto by a change of clearance and elastic deformation although very high elasticities are detrimental.
When a load is applied to the top edge, the latter is deformed and the bottom edge is deformed to a larger extent so that the stiffness of the piston skirt is adversely affected. The stiffness of the skirt is the resistance of the skirt to a deformation by a force. When a load applied to its top edge causes the piston to tilt about its top edge, the operating clearance at the bottom edge will be increased by the resulting resilient excursion. At the same time, the inclination of the piston will be increased so that head impacts may occur and the skirt noise may be increased. Similar remarks are applicable to the tilting about the bottom edge of the skirt.
This effect occurs increasingly in conventional expansion-controlled pistons because the deformation is increasingly transmitted from the stiff top edge of the skirt to the more elastic bottom edge of the skirt.
The expansion control characteristic of the piston depends on the coefficient of expansion and on the temperature gradient between the head and skirt of the piston. The coefficient of expansion and the temperature drop differ for each piston type and are influenced by structural features and engine features.
The straight-line motion of the piston depends decisively on the cooperation of the piston clearance, the expansion control action and the deformation of the skirt. A compromise must always be found between the requirements regarding an adequate strength depending on shape, smooth running, freedom from seizing, oil consumption, friction loss and weight because the stiffness and expansion control action partly influence each other.
The motion of a piston depends mainly on its clearances during operation. For this reason it is of prime importance to provide a piston which has almost the same clearances under all operating conditions. In the expansion-controlled pistons used for this purpose, the natural expansion of the skirt in the direction of movement is reduced. Known pistons of this kind have a transverse slot, which separates the head and the ring zone from the skirt. Some of these pistons have a skirt provided with a steel insert. Such pistons have a satisfactory expansion control action but their use is restricted by the fact that their strength depending on shape is inadequate.
Owing to considerations relating to strength and temperatures, motors for higher loads are provided with expansion control pistons which have steel inserts and have no transverse slot between the top edge of the shaft and the ring zone. It has been found that disadvantages of these structures reside in that either the expansion control action is poor or, where the expansion control action is locally satisfactory, there are large differences between the extent of the expansion control actions at the top and bottom edges of the skirt. Particularly where ring- or band-shaped inserts are provided near the top end of the skirt, even an inverse expansion control action has been detected in the lower part of the skirt, i.e., the expansion is larger than that which is due to the natural expansion of the base material. None of these designs permits a separate influence to be exerted on the expansion control action at the top and bottom edges of the skirt.
To permit such pistons to be designed so that they will not seize under full load, the contour to which the piston is ground is locally recessed to a larger extent or the piston is installed with a clearance which is larger than that of the slotted expansion-controlled pistons. These measures result in larger clearances during cold starting and operation under partial load so that the straight-line motion and noise of the piston will be adversely affected. (It is an object of a good expansion-controlled piston to ensure that the clearance will be minimized throughout the speed and load ranges.) Whereas an expansion-controlled piston having steel inserts has more favorable clearance than an all-aluminum piston, the former is heavier. There is a tendency to increasingly limit the piston weight because it calls for mass balancing. For this reason it is a further object to provide an expansion-controlled piston which is light in weight.
Tests have shown that the friction losses of the engine can be decreased by 3 to 5% of the useful horsepower of the engine if the surface area of the piston skirt is decreased.
In view of the foregoing considerations, it is an object of the invention to provide a piston where coefficients of expansion at the top and bottom edges of the piston skirt can be separately controlled, and an adjustment of predetermined coefficients of expansion at the top and bottom edges of the skirt can be accomplished.
It is a further object to provide such a piston where direct propagation of a deformation between the top and bottom edges of the piston skirt is prevented.
It is a further object to provide such a piston where the skirt surface area between the top and bottom edges of the skirt is decreased.